1. Field of the Invention
The present invention relates to an optical pickup apparatus for recording/reproducing information on/from a high density optical disk so that coma and spherical aberration due to changes in the thickness of the optical disk can be corrected, and more particularly, to an optical pickup apparatus in which a main light spot and an auxiliary light spot having spherical aberration are allowed to be formed on an optical disk and aberration can be corrected based on the main light spot and the auxiliary light spot received by a photodetector.
2. Description of the Related Art
In general, an optical pickup apparatus records or reproduces information recorded on or from an optical disk. With high densification of optical disks, light emitted from a light source is required to have a shorter wavelength while the numerical aperture (NA) of the light increases. Thus, when the optical pickup apparatus records/reproduces information on/from the optical disk, if the optical disk is inclined, that is, if the surface of the optical disk on which information is recorded is inclined with respect to the optical axis, a coma aberration is generated due to the inclination. Also, spherical aberration is generated due to the shorter wavelength of the light source due to the high densification of the optical disk, a high NA, and a change in the thickness of the optical disk and the wavelength accompanied by the accommodation of the compatibility with a low density optical disk.
Here, since coma aberration W31 satisfies Equation 1, a coma aberration due to the inclination of the optical disk increases in an optical pickup with a high NA compared with coma aberration in an optical pickup with a relatively low NA with respect to the same inclination of the optical disk.W31∝NA3  [Equation 1]
Also, the recording capacity of the optical disk is determined by the wavelength λ of the light emitted from the light source and the NA of an objective lens, as shown in Equation 2.Diameter of focused spot∝λ/NA  [Equation 2]
Also, spherical aberration W40d generated due to a deviation Δd in the thickness of the optical disk is defined by Equation 3.
                              W                      40            ⁢            d                          =                                                            n                2                            -              1                                      8              ⁢                              n                3                                              ⁢                                    (              NA              )                        4                    ⁢                                    Δ              ⁢                                                          ⁢              d                        λ                                              [                  Equation          ⁢                                          ⁢          3                ]            
Here, n denotes the refractive index of the substrate of the optical disk and d denotes the thickness of the substrate.
Thus, according to Equation 2, when it is desirable to set the recording density of the optical disk to 15 gigabytes or greater, a light source emitting light having a short wavelength of about 410 nm and an objective lens having an NA of 0.6 or greater are required to be adopted. However, when the NA of the objective lens is increased in order to increase the recording density of the optical disk, the spherical aberration W40d generated due to the deviation Δd in the thickness of the optical disk drastically increases because the spherical aberration is proportional to NA to the fourth power, (NA)4.
Referring to FIG. 1, a conventional optical disk aberration correcting apparatus to correct coma and spherical aberration includes an objective lens 3 for primarily focusing incident light and a focusing lens 5 for secondarily focusing the light focused by the objective lens 3 to form a spot on an optical disk 1.
When the optical disk 1 is inclined in one direction, coma aberration is corrected by driving the focusing lens 5 in the inclined direction. Also, when the spherical aberration due to the thickness deviation Δd of the optical disk 1 is to be corrected, as shown in FIG. 2, the spherical aberration is corrected by adjusting an interval between the focusing lens 5 and the objective lens 3.
In the conventional optical disk aberration correcting apparatus having the above structure, since it is necessary to drive the objective lens and the focusing lens in a direction to control the tracking and focusing of a light spot, drive the focusing lens to be inclined, and drive the objective lens and the focusing lens to adjust the interval therebetween, the structure of the actuator is complicated.